Solids. The amount of inorganic material in the paste, i.e.,
solids, must be controlled.

Fineness of grind. Both noble metals and copper are soft. Even
though the manufacturer restricts the particle size of the ingredients he uses
to 10 microns or less, agglomerations or coining on the three roll mill may
result in the presence of a small amount of larger particles in the paste.
This material could collect in the screen and interfere with good pattern
definition.

Use testing should be performed by the MIB manufacturer as part
of both qualification and acceptance testing to provide assurance that the
separate materials are compatible and that the materials and the actual MIB
processing are compatible. To perform this use test, either or both coupons
and representative MIBs may be used. Regardless of which is used, the furnace
loading during firing should represent the expected level for production.
Tests that normally are performed on the coupons and/or test MIBs are:

• Conductor adhesion

• Dielectric leakage

• Conductor resistance

• Insulation resistance

• Pad solderability

• Biased humidity

• Conductor integrity

• Quality of fired print

• Dielectric constant

These use tests are also performed on MIBs as part of normal
production line control.

Thick Film Paste Diagnostics

Many of the potential problems with the use of dielectric and
conductor pastes for MIB applications are common to both the copper and noble
metal systems. This section discusses each of these problem areas associated
with a particular material. In addition, problems with nitrogen-fired copper
and dielectric pastes that do not exist with the noble metal systems are
described, from in-coming procedures through qualification.

Nitrogen-fired copper and dielectric pastes must be kept free
from dust and fiber contamination. Dust and fiber do not burn out completely
during nitrogen atmosphere processing. The result is lack of conductor
adhesion, darkening of the copper, and considerable undersintering of the
copper in the area of contamination. The dielectric will appear darker and
will not sinter well. Dust can accumulate in paste if the paste is repeatably
returned to the original container after each use. Apportioning amounts into
smaller containers avoids this problem and is the recommended procedure.

A test for contamination of the paste is to squeeze an ounce or
two of dielectric or conductive paste through a 325-mesh screen to check for
the level and type of residue. Using this technique, one may find coined metal
and fibers as well as other contaminants.

As pastes are being evaluated for acceptance
and qualification, physical and electrical characteristics that do not fall
within acceptable limits may be attributed to the material's paste
characteristics. Paste characteristics and the correlation of these
characteristics with the observed fired characteristics are shown in Table
4-7. A particular fired module problem often can be correlated with the
initial paste characteristics. Table 4-8
shows some known correlations, identifying paste characteristics that may
be at fault when the coupon or specimen circuit tails a physical, electrical,
or environmental test.

TABLE 4-7. EFFECTS OF FIRED
PARAMETERS ON PASTE PROPERTIES

MATERIAL

PASTE PROPERTIES
VISCOSITY PERCENT
FINENESS
SOLIDS OF
GRIND

Inner layer conductor

Surface print conductor

Dielectric interlayer

Surface seal---overglaze

Conductor signal pattern accuracy and uniformity

SmoothnessGood print definition

Good via resolutionFree of pin holesGood
printability

Pinhole freeHolds print resolution at edges of conductor
pattern

Fired thickness adequate to maintain
conductivity specification

May be reduced to meet lower print viscosity needs

Must meet fired thickness specifications

Thickness control needed to reduce surface stress

Particles of a size

that may catch in

the stensil screen

will interfere with

the quality of the

printed
pattern

TABLE 4-8. PASTE
CHARACTERISTICS THAT MAY CONTRIBUTE TO FAILURE OF A PARTICULAT
TEST

Solder paste is composed of several constituents in addition to
the metal alloy. Each of the constituents performs a specific function. The
metal alloy, of course, becomes the filler and electrical path between circuit
and component. The vehicle is composed of the flux, be it rosin or resin, a
solvent, and, possibly an activator. This activator will assist in the oxide
removal and enhance solderability, and could also contain some modifiers. The
vehicle composition determines the viscosity and drying characteristics of the
paste, and combined with the powder, its flow and deformation characteristics
(rheology).

The alloy is powdered and each individual particle may have a
shape approaching spherical, or nonspherical, even to the degree of resembling
"bar bells." The shape, size, and consistency of the powder is the major
factor in the paste's ability to be screen-printed with a certain size screen,
how fine a feature can be stenciled, how small a syringe needle can be used
for dispensing, and whether or not these apparatus will quickly foul during
use. The particle size and shape have effects on the potential for surface
oxidation as some forms contain more surface area than others.

A correctly specified and supplied paste will perform well and
repeatably through application, component placement, dry, reflow, and clean.
Each of the operations, along with its attendant use parameter, can have an
effect on the formulation and successful use of a solder paste. The first step
in the selection of a solder paste is to communicate with the manufacturer.
The manufacturer should be made aware of the following parameters by the
user:

• The temperature for reflow operation. This will be used to
determine the alloy.

• The flux system (refer to restrictions of
Federal Specification QQ-S-57 - see Appendix B
). Note that the same flux should be
used for any touch-up - a supply can be ordered with the solder paste.

• The intended method of deposition - screen, stencil, syringe
manifold, etc. A specification of screen mesh, smallest stencil feature,
needle size, etc., must also be made to ensure the proper powder size, and
shape.

• The deposited thickness and aspect ratio desired. This will be
somewhat influenced by the metal content.

• The "tacky time" or the time between deposition and component
placement. This parameter could range up to five days.

• Method of reflow. If condensation heating (vapor-phase) is to
be the reflow mechanism, a predry may be required to drive off volatiles and
moisture to aid in preventing solder ball formation and component
shifting.

• The solvents intended to be used for cleaning. This is to
ensure that the flux residue can actually be cleaned with the specified
solvent.

• All of the solderable materials to be joined. Include the pad
material of the MIB and all components, especially the end cap material of
capacitors and resistors, and whether the surface-mounted devices have been
pretinned or not.

In addition to describing the user materials and process as
fully as possible to the manufacturer, several other items may be desirable or
required from the manufacturer by the user:

• Lot traceability of the paste and all of its constituent
materials

• Manufacturer-recommended storage temperature and
environment

• Manufacturer guarantee of shelf life referenced to the
manufacturer's date of manufacture